This invention relates to a tapping machine wherein a spindle carrying a screw tap is reciprocated while being rotated to form a female screw in a work, and more particularly to a tapping machine capable of halting the drive of an electric motor, which is to drive the spindle, when the temperature of the motor has risen exceeding a predetermined value.
As tapping machine of this kind, one wherein the motor is halted or stopped immediately after the temperature of the motor has exceeded during the tapping operation a predetermined value, and another wherein the motor is reversely rotated soon after the taking place of the same situation for withdrawing the spindle to the start position or original position accompanied by the complete stoppage of the motor, are well known.
In the former machine the motor is halted with the screw tap being engaged with the work, the motor is therefore obliged to meet with a very large torque when the motor is re-driven after it has been cooled, sometimes leading to a complete incapability of driving. It often necessitated the machine operator to remove the work from the screw tap and to return the spindle to the original position. In such an instance the screw tap must be engaged again with the work which has been machined midway for finishing the tapping operation. Such re-engagement of the screw tap and the work in the middle of the machining is very difficult, so an exact female screw forming is next to impossible. It is a great disadvantage of the former prior art.
The latter one is similarly disadvantageous in its difficulty of rendering the screw tap accurately engage again with the half formed female or internal thread. This is also far from being a satisfactory tapping machine.